But while Mkhitaryan CAN turn out for the Gunners in the UEL, Aubameyang will be INELIGIBLE.

The Gunners currently sit sixth in the Premier League, five points behind Liverpool in the fourth and final Champions League berth. It's looking likely that they will have to emulate Man United and win the Europa League to earn a return to European football's top table.

It means Arsenal are going to have a lot of games to negotiate, starting with next month's round-of-32 tie against Ostersunds.

Who can clubs register for the knockout rounds?

Clubs can add three new players to their squads.
- Only one player can have played in the group stage of the other competition to which the club is currently in (so a Europa League player can move up to the Champions League, and vice versa)
- Only one player can have played in just the qualifying rounds of either competition that season.
- Any addition can have played no European football this season.

Who is not eligible?

Players who have:
- Played in the same competition for another club
- Player for another club that is currently in the same competition

INELIGIBLE: Pierre-Emerick Aubameyang

The rules around player eligibility can get somewhat confusing here, so to avoid getting bogged down in the intricacies of the regulations (43.02) let's put it simply: Borussia Dortmund and Arsenal are in the same competition.

- Pierre-Emerick Aubameyang played for Borussia Dortmund in the Champions League group stage
- Borussia Dortmund have dropped into the Europa League
- Arsenal are also in the Europa League
- Thus, even though Aubameyang has not played in the Europa League this season, he cannot play for Arsenal in it because Dortmund are now in the same European competition and he has already played in the group stage of a European competition.

Henrikh Mkhitaryan failed to take his chance in a rare start for Man United. Martin Rickett/PA Images via Getty Images

ELIGIBLE: Henrikh Mkhitaryan

Just like Aubameyang, Henrikh Mkhitaryan has moved from a club playing in the Champions League to Arsenal in the Europa League.

However, as Man United -- unlike Dortmund -- are still in the Champions League, Mkhitaryan is thus free to play for Arsenal in the Europa League

Could Arsenal have registered BOTH Aubameyang and Mkhitaryan if eligible?

No, as clubs can only name one player who has already played in the other European competition group stage. A famous example came in 2009 when Real Madrid signed both Lassana Diarra and Klaas-Jan Huntelaar -- they chose to leave out Huntelaar.

Arsenal would have had to make a choice between Aubameyang and Mkhitaryan for the UEL. But as Aubameyang is ineligible, it will be Mkhitaryan who is registered.